$oMerger->addPDF($file, 'all'); //Add all pages
$oMerger->addPDF($file, [1]); //Add page one only
$oMerger->addPDF($file, [2]); //Add page two only
$oMerger->addPDF($file, [1, 3]); //Add page one and three only

...merge files together but add blank pages to support duplex printing: